Nobody ever talks about it but why not add Louisville anyways to stay at 11. Big 10 did it for years and it seemed to work fine.
Because the next TV deal is probably going to "incentivise" the Big 12 moving back to 12 or even 14. And 11 is actually a ridiculous number because you are caught in no-man's land - too many teams to do a true round robin (10 of your 12 games in football and 20 basketball games would need to be devoted to it - not going to happen) and not enough teams to get the championship game.
And no, it really didn't work that well for the Big 10 all those years. It was always awkward for the conference schedule-wise both in basketball and football, and the basketball tournament was unbalanced and also awkward. The only reason why they went ahead with it was because they wrongly assumed that Notre Dame was eventually going to join the conference, pushing them to 12. It was never the Big 10's intent to stay at 11.